Field of the Invention
The present invention relates to a tyre for motorcycles.
Description of the Related Art
The tyre of the present invention preferably is a tyre for wheels of high performance motorcycles, i.e. motorcycles capable of reaching speeds even higher than 270 km/h. Such motorcycles are those belonging to the category typically identified with the following classifications: hypersport, supersport, sport touring, and for lower speed indexes: scooter, road Enduro and custom.
The following definitions apply in the present description.
The terms “tyre for motorcycles” and/or “tyre for motorcycle wheels” are used to indicate tyres having a high curvature ratio (typically higher than 0.200), capable of achieving high camber angles during the cornering of the motorcycle.
“Camber angle” is used to indicate the angle between the equatorial plane of the tyre mounted on the motorcycle wheel and a plane orthogonal to the road surface.
“Equatorial plane” of the tyre is used to indicate a plane which is perpendicular to the rotation axis of the tyre and which splits the tyre into two symmetrically equal parts.
“Curvature ratio” of the tyre is used to indicate the ratio between the distance comprised between the radially higher point of the tread band and the maximum tyre chord, and the.
same maximum tyre chord, in a cross section thereof. The curvature ratio also identifies the so-called “transverse curvature” of the tyre.
The terms “radial” and “axial” and the expressions “radially inner/outer” and “axially inner/outer” are used with reference to the radial direction and to the axial direction of the tyre. The terms “circumferential” and “circumferentially”, on the other hand, are used with reference to the direction of the annular development of the tyre, which corresponds to a direction lying on a plane parallel to the equatorial plane of the tyre.
The term “elastomeric material” is used to indicate a composition comprising at least one elastomeric polymer and at least one reinforcement filler. Preferably, such composition further comprises additives such as, for example, a cross-linking agent and/or a plasticizer. Thanks to the presence of the cross-linking agent, such material may be cross-linked by heating.
The expression “reinforcing cord” is used to indicate an element consisting of one or more elongated elements incorporated in a matrix of elastomeric material. Depending on the cases and to the specific applications, the above elongated elements are made of a textile or metal material.
The expression “elongation at break” of a reinforcing cord is used to indicate the percentage elongation at which break occurs, as determined by the BISFA E6 method (The International Bureau For The Standardization Of Man-Made
Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition).
The expression “part load elongation” of a reinforcing cord is used to indicate the difference between the percentage elongation obtained by subjecting the cord to a tensile strength of 50 N and the percentage elongation obtained by subjecting the cord to a tensile strength of 2.5 N. The part load elongation is determined by the BISFA E6 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition).
The expression “reinforced strip-like element” is used to indicate an elongated composite having a cross-sectional profile with a flat shape and comprising one or more reinforcing cords extended parallel to the longitudinal development of the product and incorporated in, or at least partly coated by, at least one layer of elastomeric material. Such reinforced strip-like element is also commonly called “band-like element”.
Tyres for motorcycle wheels have a high transverse curvature so as to provide an adequate contact surface with the road surface when the motorcycle is inclined to turn a bend.
In addition to bearing the motorcycle weight in all the running conditions (thus including the weight of the driver, any passengers and any loads), such tyres should ensure driving stability, controllability, directionality, comfort, road-holding, besides high mileage and regular wear.
High performance tyres, which are suitable for being mounted on large piston displacement (e.g. 1000 cm3 or higher) and/or high power (e.g. 170-180 HP or higher) motorcycles, should provide an excellent adhesion to the road, so as to effectively discharge the high driving torque to the ground both when driving on straight roads and during accelerations when exiting a bend, as well as ensure an adequate response to the side forces on bends and an effective braking force.
The behaviour of tyres for motorcycles, especially on bends and in particular in the case of tyres for high performance motorcycles, highly depends on the particular type and shape of the belt structure. In fact, the belt structure is configured to transfer to the carcass structure the side and longitudinal stresses the tyre is subjected to because of the contact with the road surface, thus contributing to imparting the desired features of structural resistance, performance (i.e. adhesion, driving stability, controllability, directionality, road-holding) and comfort to the tyre.
The particular type and shape of the belt structure has a considerable impact on the weight and/or production cost of the tyre.
The use of belt structures of the so-called “crossed” type and belt structures of the so-called “zero degree” type is widespread in tyres for motorcycles.
Belt structures of the “crossed” type typically have two belt layers arranged radially one on top of the other. Each belt layer comprises a plurality of reinforcing cords arranged parallel to each other and made of a textile or metal material, more typically textile in tyres for motorcycles. The belt layers are reciprocally arranged so that the reinforcing cords of the first belt layer are tilted with respect to the equatorial plane of the tyre, whereas the reinforcing cords of the second layer are also tilted, but they are oriented on the opposite side with respect to the reinforcing cords of the first layer with reference to the equatorial plane of the tyre.
Belt structures of the “zero degree” type typically comprise a plurality of reinforcing cords made of a textile or metal material. Such reinforcing cords are wound on the carcass structure according to coils arranged next to one another in the axial direction and oriented substantially parallel to the equatorial plane of the tyre or tilted with respect to said equatorial plane by a very small angle.
EP 0 461 646 describes a tyre for motorcycles comprising a belt structure of the zero degree type formed starting from a plurality of high elongation reinforcing cords (also called “HE cords”). Each reinforcing cord comprises a plurality of strands twisted together, each strand in turn comprising a plurality of metal wires twisted together.
EP 2 127 903 describes a tyre for motorcycles comprising a belt structure of the zero degree type formed starting from a steel-rubber composite material obtained by incorporating into the rubber multiple steel elements individually shaped as a spiral, without the above steel elements being wound spiral-wise with each other.
EP 1 284 318 describes a tyre for motorcycles comprising a belt structure of the zero degree type formed starting from a plurality of metal wires having an elongation at break comprised between 3% and 7% and a breaking load not lower than 2700 N/mm2.
EP 1 213 159 describes a tyre for motorcycles comprising a belt structure of the zero degree type formed starting from reinforcing cords comprising each a plurality of metal wires twisted together leaving a central gap between said metal wires extending by the entire wire length, such gap being occupied by the elastomeric material mixture after the vulcanization. The metal wires of each cord are from two to seven. The reinforcing cords have an elongation at break comprised between 3% and 6%.